The present invention relates to a method for the growth of nitride based semiconductors such as GaN, InN, AlN and other alloys. The invention relates also to an apparatus for use with such a method.
GaN and its related compound semiconductors are the key materials for blue-green light emitting diodes (LEDs) and semiconductor lasers. The preparation of high quality epitaxial layer of this material has been intensively pursued for over twenty years (see J. Pankove, U.S. Pat. No. 3,864,592; H. Kobayashi et. al., U.S. Pat. No. 4,473,938; K. Manabe et. al., U.S. Pat. No. 4,911,102). The main obstacle to the achievement of high efficiency light emitting diode is the preparation of highly conductive p-type GaN. Low energy electron beam irradiation (see H. Amano et. al., "P-type conduction in Mg-doped GaN treated with low-energy electron beam irradiation (LEEBI)," Jpn. J. Appl. Phs. 28, L2112-L2114, 1989) and thermal annealing (see S. Nakamura et. al., "Hole compensation mechanism of p-type GaB films," Jpn. J. Appl. Phys. 31, 1258-1266, 1992) have been used to activate the Mg-doped GaN epilayers. Dehydrogenation was proposed to be the critical step since highly conductive p-type GaN films could be grown in a hydrogen-free ambient (see M. E. Lin st. al., Appl. Phys. Lett. 63, 932-934, 1993; C. Wang and R. F. Davis, Appl. Phys. Lett. 63, 990-992, 1993; M. Rubin et. al., Appl. Phys. Lett. 64, 64, 1994). This invention presents the design of a chemical beam epitaxy system and the method for the epitaxial growth of large area epitaxial layers of GaN and its related compounds.
Conventionally, there are two most effective methods to prepare GaN epilayers, namely molecular beam epitaxy (MBE) and organometallic vapor phase epitaxy (MOVPE). Because molecular nitrogen is too inert to react with Ga, radio-frequency plasma-assisted growth methods (see W. E. Hoke, P. J. Lemonias and D. G. Weir, "Evaluation of a new plasma source for molecular beam spitaxial growth of InN and Gan films", J. Crystal Growth III, pp. 1024-1028, 1991 and M. Liu, A. C. Frenfel, J. G. Kim, and R. M. Park, "Growth of zinc blende-GaN on .beta.-SiC coated (001) Si by molecular bem epitaxy using a radio frequency plasma discharge, nitrogen free-radical source", J. Appl, Phys, 74, pp. 6124-6127, 1993) and microwave plasma-assisted growth methods (see C. H. Carter, Jr., U.S. Pat. No. 5,210,051, May 1993 have been most widely used for MBE while NH.sub.3 is used for MOVPE (K. Manade et al., U.S. Pat. No. 4,911,102, March 1990). The group III sources used are elemental metals evaporated from effusion cells for MBE and vapors from metal-organic compounds for MOVPE, respectively.
Using a plasma-assisted growth method, MBE shows only limited success.
Though the material obtained by MBE is of reasonably high quality, the growth rate is less than 0.6 um/h. This slow growth rate is attributed to the limited reactive nitrogen atom or ion flux provided by the plasma source for the growth of stoiohiometric films. The usable flux in a MBE system is determined by the efficiency of the nitrogen source and the distance between the substrate and the source. Because of the divergent nature of the nitrogen beam, the usable flux decreases dramatically as the substrate is located far away from the nitrogen source. Further, increasing the nitrogen rate into the plasma source raises the growth pressure above 10.sup.-4 Torr and reduces the mean free path of the reactants, which deteriorates the growth rate. Modification made on the conventional MBE chamber to accommodate the plasma source can not avoid the difficulties mentioned above which shorter distance between the substrate and the plasma source will either affect the uniformity of the epilayer or damages the epilayer when an electron cyclotron resonance (ECR) source is used.
Currently, the only method that is employed for the production of GaN LEDs is MOVPE. Because high quality GaN can only be grown at 1000.degree.-1100.degree. C., thermal convection and gas phase pre-reaction have impeded the success of conventional MOVPE method. A two-flow reaction chamber has been proposed (see S. Nakamura, Jpn. J. Appl. Phys. 30, L1705-L1707, 1991) to overcome these barriers and produce high brightness blue LEDs. However, this method has disadvantages. Though the thermal convection and gas phase pre-reaction can be somewhat suppressed by reducing the pressure in the reactor, a large amount of pressing gas is still necessary for the growth of GaN. Morever, the nitrogen source, i.e. NH.sub.3, used in this method produces a great amount of hydrogen which is detrimental to the p-type GaN. Therefore, post-annealing on the MOVPE grown epilayers above 700.degree. C. in a nitrogen ambient to activate p-type dopant Mg is necessary. Moreover, the manner of the introduction of the reactant gas to the substrate hinders uniform growth of epilayers over a large area, e.g. greater than two inch diameter.